By Jane O'Brien
BBC News, Washington
Karl Rove's resignation is not in itself a surprise - he discussed leaving his post as deputy chief of staff as early as last summer - and with George W Bush's presidency drawing to a close there was arguably little left for the political strategist to do.
But many White House watchers are wondering about the timing of his departure.
According to Mr Rove, he wants to spend time with his family.
But some Democrats have suggested that he is walking away from congressional investigations questioning his involvement in key government decisions that may prove to have been illegal.
He was implicated in the CIA leak case that dogged the administration for several years, having been cited as the person who revealed the identity of Valerie Plame, a covert CIA agent whose husband was critical of the administration's decision to go to war in Iraq.
But although he testified five times before a grand jury, he was never charged with any criminal offence. Instead, another top aide, Lewis 'Scooter' Libby, was convicted of perjury and of obstructing the investigation.
Mr Rove is also accused of being behind the decision to sack eight federal attorneys. The Democratic-controlled Congress says the dismissals were politically motivated, but Mr Rove has claimed executive privilege and has refused to testify.
"The list of senior White House and Justice Department officials who have resigned during the course of these congressional investigations continues to grow," said Senator Patrick Leahy, Democratic chairman of the Senate Judiciary Committee.
"There is a cloud over this White House, and a gathering storm."
Karl Rove is one of the most powerful men at the White House. He has been at the centre of every political and strategic decision of the Bush administration and the president's closest political ally.
Karl Rove and George W Bush have worked together since 1993
He's been dubbed "Bush's Brain" and "The Architect" for his role in securing White House victory for Mr Bush in 2000 and for masterminding the campaign that kept him there in 2004.
At a press conference to formally announce his resignation, the president was visibly upset.
"I would call Karl Rove a dear friend," he said.
"We've known each other as youngsters, interested in serving our state, and we worked together so we could be in a position to serve this country."
The relationship between the two men dates back to the 1970s, when Mr Rove first entered Republican circles.
He went on to become an adviser to George Bush senior and helped the younger George Bush become governor of Texas in 1994.
"He has been Bush's personal strategist his entire career," said Professor Clyde Wilcox, an expert on US politics at Georgetown University.
"He has been so central to co-ordinating their political strategy, their policy strategy, their defence against the press and against Congress, that he will be a pretty major loss."
That loss is likely to be felt in September, when Mr Bush faces one of the toughest tests of his presidency.
US commanders will give a progress report on the controversial "surge strategy" - the escalation of troops in Iraq - and Congress will debate whether it is good enough to continue funding the war.
Mr Rove's guiding hand will also be missed as the administration continues to battle against poor ratings and congressional investigations.
With his resignation there is the sense that an era of US politics is ending. In his parting remarks Mr Rove himself said he was "grateful to have been a witness to history".
"It has been the joy and the honour of a lifetime," he continued, his voice quivering at times. "But now is the time. ... At month's end I will join those whom you meet in your travels, the ordinary Americans who tell you they are praying for you."
His critics have a less favourable postscript.
One of the Democratic presidential front runners, Barack Obama said: "Karl Rove was an architect of a political strategy that has left the country more divided, the special interests more powerful, and the American people more shut out from their government than any time in memory."
Nobody questions that Mr Rove was a brilliant political strategist, the man who created the Bush presidency.
But his final legacy may be more difficult to define. To his critics he will always be an evil genius; to his intimates, an irreplaceable friend.